Huntington's disease (HD) is caused by the expression of the toxic mutant huntingtin protein (mthtt). Huntingtin is also the most salient molecular target for disease modifying therapy and indeed treatments that target its production, processing, or turnover are under development. Measurement of mthtt has broad potential as a biomarker. Its levels in blood could correlate to disease activity, severity, or stage; changes in its levels could provide a pharmacodynamic marker for therapies directly or indirectly modulating huntingtin. We recently helped develop a bioassay for measuring soluble mutant huntingtin levels utilizing time-resolved Forster Resonance Energy Transfer (trFRET). In preliminary studies applying the assay to blood in HD mice and human HD, it is highly sensitive for soluble mutant huntingtin in tissues and cellular fluids and detects it in blood from premanifest and manifest HD subjects. Through our NIH supported Program Project (P01- NS058792, Biomarkers for Huntington's Disease: accelerating the development of therapies), we are accumulating blood samples from large populations of controls, premanifest HD, manifest HD subjects, and HD subjects participating in therapeutic trials. In this application, we are seeking to optimize and validate this assay, ready it for GLP use, and assess how blood levels of huntingtin change with the development of symptoms, with progression of symptoms, and in response to a potential neuroprotective treatment. This accelerated review PAR is appropriate because it depends on subjects and blood samples from active single and multi-center clinical research studies supported by NIH including REVEAL-HD, PRECREST, PHAROS, and CREST-E observational and therapeutic trials.